Sofia - welcome to the Forge! I like your thinking, but here's some clarification, insofar as I understand what's going on:

Willow is in the middle of a D&D campaign in which she's moving the events from the European-analogue standard D&D environment to a Middle-East-like area of the world. For this reason compatibility with the standard D&D cosmology is of major importance for her whole idea; the point is not to emulate real history of the region, but to take on color and ideas for setting and situation from real world history. The artistic impact will come from being faithful to the context of 4th edition D&D cosmology and presenting any original additions within that context. For this reason a loose cherry-picking approach to the real world history is better than pedantry here.

(I felt it necessary to repeat where Willow is coming from here - for some reason there's been a lot of challenge for her starting point in this thread, as we can see by rereading from the beginning. It's a pretty straight-forward topic, no need to bring in any more complex thought than D&D settings invest in depictions of medieval feudalism.)

Ron has some pretty good ideas on the different real-world situations of the region. One thing I might be interested in for a D&D twist like this would the idea of Oriental Orthodox Churches, which in your treatment might be the local Pelorian faith, a subsumed minority next to the more visible Bahamut/Erathis pairing. Perhaps the thing with Pelorians locally could be that they're weak and perhaps even a bit persecuted, but they're also not in communion with the big and powerful Pelorian cults back in not-Europe because of some stupid theological disagreement. This could end up as just color, or if you've got Pelorian characters or NPCs in the campaign, then they might have to decide what they think about these distantly related eastern Pelorians.

Hmm... I feel a summation coming on:

Perhaps the majority of the population worships Bahamut and Erathis as the male and female principles of civilization - or maybe animal and human, considering how Bahamut is not a human god. Bahamut would be the Jihad god, while Erathis is the not-Persian goddess of civility. The two could have many common myths involving dragonriding and dragonlances, and considering them politically equal could be a touchy point for the dominant culture, which is a rough amalgam of earlier not-Arab and not-Persian cultural spheres; challenging the association of the two or trying to depict one as superior to the other could be seen as having political significance. Alternatively, go full-hog into Sunni vs. Shia on it, with some polities actively subsuming one partner to the other. Could even presume that Bahamutian areas have a patrilinear inheritance, while Erathians go matrilinear - a real mess.

Meanwhile, a similar but much older cult set-up could exist among a specific ethnicity of not-Jews, worshipping Moradin and the Raven Queen (known by some given name among these people, likely) as man and wife. This would be a much smaller religious community than those following the traditions of Bahamut and Erathis - perhaps they're called the "Little Pair" in colloqual speech, even. The local understanding would be that the Little Pair is a much older and, to scholars, more credible religious tradition, with a moral priority that can get embarrassing for the now dominant new-comer "Great Pair". The similarities and differences between the two pairs of gods would presumably be an important theological topic. I guess some would even argue that Moradin and Bahamut are one and the same being.

Pelor would be a strange-seeming god in this overall set-up, for he is not paired with a female deity. Might consider adding finiteness (he was once a man) and an eschatology (one day he'll take revenge on all the bullies) if you feel like it. And as intimated above, even while Pelor is powerful in distant foreign lands, his is locally a minority religion with dubious ties to Pelorian cults in other lands (perhaps because they don't want to offend the dominant other cults of the area) and even more dubious ties to the local religious traditions; Pelorians claim that their god is the son of Moradin or father of Bahamut or other such things, but who's going to believe it?

Melorans are definitely local pre-duotheism (that's what those god-goddess pairings amount to, even if everybody's technically polytheistic and considers gods outside their own cult real) pagans. They remember the time when the land was not a sandy desert, but rather green with grasses and forests, the time before civilization and over-grazing. This Paradise, as they call it, is still to be seen on the banks of the Great Rivers, a handful of streams around which much of the population base is centered. Melora, they believe, is the most important of the deities because it is her riverine presence that sustains the world and the natural spirits which they also revere. Oasiss are also her thing, and usually have a shrine to her.

Ioun can have that Bahá'í schtic, perhaps - and Sikh, too. As we know, all Sikh have the same name, and they do crazy martial arts, and they believe in the teachings of successive gurus. Sounds like the god of knowledge to me, especially combined with the idea that the Iounnite revelation has not been completed yet - they're still waiting for a couple of gurus to show up and complete the teachings. Perhaps they don't even consider Ioun a god proper, but rather use the name for the combined total of divine revelation, of which the mythologies of other cults are a minor part. So an Iounnite might actually be Bahamut-worshipper or Moradin-worshipper, he just wouldn't do too much of that in practice because he has this other body of knowledge that says that he doesn't have to worship. Like a combination of Gnostic and Dervish relationships to the main faiths of Christianity and Islam.

The evil gods like Vecna and such should have plenty of room in the demonology. I'd probably pick Vecna as the Satan-equivalent just because he's cooler than the rest of the bunch. So he was the advisor to Moradin, betrayed Bahamut and killed (or perhaps seduced) Pelor in his human form, depending on who you ask. Eevil deevil.

Hmm... probably too clean, laid out like that. In practice people would of course know fuck-all about what their cults actually teach, and of course most of the cults most of the time would be open to worship at multiple temples - after all, all the good gods are all super-friends with each other, right? Better that they worship the old and frail Moradin than Vecna, amirite?

Eero, you keep coming up with awesome coolness! If only I had read that insight a week ago.

Anyway, here's the writeup I ended up coming up with:

A Guide to the House of Peace

The House of Peace is a land of civilization, of glorious sights, and wondrous vistas. The land is a harsh mistress, but the people are wise, faithful, and beautiful. It is a land of thousands of years of history.

Currently the House of Peace is divided into a number of kingdoms, the largest of which is the Caliphate. These kingdoms are loose alliances of various city states, points of light in a hostile land filled with desert raiders, strange monsters, and vengeful devils and djinns.

The Old Kingdom

A millennium ago, the House of Peace was united under the Old Kingdom. The King was wise and just, and his people prospered. Treachery, disaster, and warfare brought the Old Kingdom down, but its people, most notably Dwarves, Eladrin, and Dragonborn, continue to practice their faith, and believe that one day the Old Kingdom will be restored.

The Caliphate

Almost half the House of Peace is united under the rule of the Caliphate, a vast empire ruled by Caliph Bethel, a Priestess-Empress who utterly dominates the kingdom, and is considered an honorary Exarch of the gods of the Caliphate.

Jalalabad

Almost at the center of the House of Peace is the city of Jalalabad, an ancient city and center of trade. It is set almost on the source of the Tiger river, which flows from the Perilous Moon Mountains. Defensible but a key trade location, Jalalabad was the center of a successful kingdom, but was on the wane. The vizier Musafir bin Tafrin, a corrupt and feared man, recently used his wizardry might to drive back the armies of the Caliph, but there's no telling when they will return.Faiths of the House of Peace

A common belief is that the true names of heavenly figures are divine, and not to be spoken aloud by mortal lips. Thus, when referring to gods, southerners will use euphemisms or titles. If they do use the true name of their god, it is a foul oath- considered blasphemy by many.

Gods of the Old Kingdom

The Old Kingdom worshiped a pantheon of gods symbolizing peace and prosperity: The High-Father (Moradin), his wife, the Raven Queen, his brother, the Celestial Cultivator (Corellon), and her parents, the Immutable Sun (Pelor), and the Heavenly Mother (Erathis).

According to legend, the Raven Queen and the High-Father separated, ending their heavenly marriage. Whose fault is to blame varies from story to story. Dwarves tend to take the side of the Raven Queen, Dragonborn the High Father. The faith is also popular with Eladrin, to tend to worship the secondary deities of the Celestial Cultivator and the Immutable Sun.

Gods of the Caliphate

The faith of the Caliphate exalts a pantheon of five gods united to bring the whole of the world under the sway of the Caliph: the Heavenly Mother and Supernal Sovereign (Erathis), the Most Merciful and Exalted Dragon (Bahamut), the Unstoppable Sword (Bane), the Veiled Lady (Ioun), and the River of Life (Melora). Tieflings often follow the ascetic practices of the Veiled Lady, and Genasi commonly are found as civilized worshipers of the River of Life. While few, many Deva believe in the goals of the Caliphate, and align themselves with the Exalted Dragon or the Supernal Sovereign.

Nomad Wanderers

Above all else, nomad wanderers (especially halflings) pray to the River of Life (Melora), for she grants them life, and the uncanny ability to find oasis and wells in the desert. But they also follow the three fates: the Fickle Fate (Avandra), the Lover's Fate (Sehanine), the Veiled Fate (Ioun), and the Final Fate (Raven Queen). Elves tend to follow the path of the wanderer.Other Gods

Followers of the Heavenly Fist (Kord) seek to emulate their patron- growing their hair long, forgoing most worldly possessions, and striving for personal physical perfection. They are a rare sort, but the path is often led by Half-Orcs and Shifters, who while rare, have the natural strength and propensity for body hair.

The Fivefold Dragon (Tiamat) is often secretly worshiped by wealthy merchants, of the Old Kingdom's faith or the Caliphate alike.

Kali-Ra (Lloth) is idolized as a dark skinned, six-armed woman, who directs her cultists to infiltrate and subvert civilized society. As she has no known epithet, her followers are often simply referred to by outsiders as 'shadow cultists.'

The King that Crawls (Torog) has the same reputation as in the north- the chained god of the Underdark. Worship of him is acceptable in the Caliphate, but only in the proper context, and to appease him.

The Old Man of the Mountain (Vecna) commands a hidden cabal of assassins, constantly at odds with the followers of Ioun.

Old One Eye (Gruumsh) commands hordes of orcs and other ravaging humanoids that threaten the cities of the South. His followers hold no qualms about speaking his name.

The Prince of Lies (Asmodeous) is feared, but he has recently gained dominion over the city-state of Jalalabad.

The Supernal Serpent (Zehir) is the source of many cults, ranging far and wide, many originating with patrons in Meztequa, where his worship is legal and enforced.

There was a strong vibe of "strange land, yet oddly similar to our own." Such as when their guide, Mobutu bin Ubuntu, the halfling scout described the land as "too many people with too few resources, clustered into the city states, beautiful gems, like Points of Light in a harsh wilderness." (To which the Drow quipped "sounds just like the Underdark.")